Lex: Check buckets on header map construction

If the number of buckets is not a power of two, immediately recognize
the header map as corrupt, rather than waiting for the first lookup.  I
converted the later check to an assert.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@261448 91177308-0d34-0410-b5e6-96231b3b80d8